class SyntaxTree::Mermaid::FlowChart

def node(id, label = " ", shape: :rectangle)

Add a node to the flowchart with an optional label. 
def node(id, label = " ", shape: :rectangle)
  node = Node.new(id, label, shape)
  nodes[id] = node
  output.puts("#{prefix}#{nodes[id].render}")
  node
end